Method for electrophoresis in liquid crystals

ABSTRACT

A method of electrophoretic movement of particles through a liquid crystal utilizes a direct (DC) or alternating (AC) electric field that is applied along the liquid crystal director (for liquid crystals with a positive dielectric anisotropy) or perpendicular to the director (for liquid crystals with a negative dielectric anisotropy). A perpendicular or tilted orientation of the liquid crystal molecules at the surface of the particle causes distortions, such that the fore-aft (or left-right) symmetry of the particle is broken. The asymmetric orientation of the liquid crystal around the particle allows both charged and neutral particles to be transported, even when the particles themselves are perfectly symmetric (spherical).

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.61/460,034 filed Dec. 22, 2010, the content of which is incorporatedherein by reference.

TECHNICAL FIELD

The present invention relates to liquid crystals. In particular, thepresent invention relates to the use of liquid crystals as a medium forelectrophoresis. More particularly, the present invention relates to theuse of DC and AC electric fields to achieve electrophoretic movement ofparticles in a liquid crystal.

BACKGROUND ART

Electrophoresis is the motion of charged dispersed particles relative toa fluid in a uniform electric field, and is typically used to separatemacromolecules (e.g. DNA and proteins), to assemble colloidalstructures, to transport particles in nano-fluidic and micro-fluidicdevices and displays. For example, electrophoresis is utilized totransport particles in micro-fluidic displays, such as that used by theKINDLE™ electronic reader, and other electrophoretic or e-ink displays.Typically, the fluid, such as water, utilized as the electrophoreticmedium is isotropic, whereby the electrophoretic velocity of a dispersedparticle in the fluid is directly proportional to the applied electricfield. Because of this linear dependence, only a direct current (DC)electric field can be used to drive the particles through the fluid.However, the use of an alternating current (AC) electric field to drivethe particles through the fluid is more desirable, as it preventselectrolysis, i.e., electrochemical reactions near the electrodesresulting in degradation of the medium. Furthermore, the use of ACelectric fields to drive the particles allows one to create steady flowsof particles, as the AC field avoids accumulation of the electriccharges near the electrodes screening the electric field. Furthermore,the direction of electrophoretic motion of the particles in an isotropicfluid, such as water, is typically parallel or antiparallel to theapplied electric field, which makes it difficult to designthree-dimensional trajectories of the particles. To move the particle ina direction that is not collinear with the direction of electric field,one needs to design the shape or to modify the surface properties ofparticles.

Therefore, there is a need for a method of moving particles by electricfield in which the driving force can be an AC field and in which thetrajectory of the particle can follow a predetermined 3D pathway. Inaddition, there is a need for a method of moving particles byelectrophoresis in a liquid crystal using direct current (DC) oralternating current (AC) electric fields. Moreover, there is a need fora method of moving particles through a nematic liquid crystal (LC),whereby the electrophoretic velocity is proportional to the square ofthe applied voltage, allowing the particles to be moved by an ACelectric field. In addition, there is a need for a method of movingparticles by electrophoresis through a liquid crystal upon applicationof an AC electric field, such that the liquid crystal orientation aroundthe particle is distorted, so as to break the fore-aft or right-leftsymmetry of the liquid crystal surrounding the particle. Furthermore,there is a need for a method of moving particles by electrophoresisthrough a liquid crystal, such that the direction of electrophoreticmotion of the particle is controlled by three factors: the direction ofthe electric field, the shape of the particle in conjunction with thelocal orientation of the liquid crystal around the particle, and anorientationally ordered nematic liquid crystal far from the particle, soas to allow three-dimensional control of trajectories of movingparticles to be constructed.

SUMMARY OF THE INVENTION

In light of the foregoing, it is a first aspect of the present inventionto provide a method for electrophoresis in a liquid crystal.

It is another aspect of the present invention to provide a method ofelectrophoretic movement of a particle comprising providing a nematicliquid crystal between a pair of plates, said nematic liquid crystalhaving a director field in which the orientation of said director fieldproximate to said plates has a first alignment orientation; disposing aparticle having a surface in said nematic liquid crystal, such that theorientation of said director field proximate to said surface has asecond alignment orientation different from said first alignmentorientation to thereby form distortions in said director field around atleast part of said surface of said particle; subjecting said liquidcrystal to an electric field, such that said distortions around saidparticle cause the velocity of mobile ions in said liquid crystal movingbetween said plates to be asymmetric; and moving said particle throughsaid nematic liquid crystal.

Yet another aspect of the present invention to provide a method ofelectrophoretic movement of a particle comprising providing a pair ofplates; disposing a polyimide layer on one of said plates; buffing saidpolyimide layer in a predetermined direction; disposing a nematic liquidcrystal between said plates, said nematic liquid crystal having adirector field, such that the orientation of said director fieldproximate to said polyimide layer has a first alignment orientation thatis in substantial alignment with said predetermined direction; disposinga particle having a surface in said nematic liquid crystal, such thatthe orientation of said director field proximate to said surface has asecond alignment orientation that is different from said first alignmentorientation to thereby form distortions in said director field around atleast part of said surface of said particle; subjecting said liquidcrystal to an electric field, such that said distortions around saidparticle cause the velocity of mobile ions in said liquid crystal movingbetween said plates to be asymmetric; and moving said particle throughsaid nematic liquid crystal along said predetermined direction.

It is another aspect of the present invention is to provide a liquidcrystal cell comprising a pair of plates adapted to be coupled to apower source; a liquid crystal material disposed between said plates,said liquid crystal material having a director field in which theorientation of said director field proximate to said plates has a firstalignment orientation; and a particle disposed within said liquidcrystal material, said particle having a surface and configured suchthat orientation of said director field proximate to said surface has asecond alignment orientation different from said first alignmentorientation to thereby form distortions in said director field around atleast part of said surface of said particle, wherein when power isapplied to said plates by said power source, said distortions cause thevelocity of mobile ions in said liquid crystal material moving betweensaid plates is asymmetric, causing said particle to move.

BRIEF DESCRIPTION OF DRAWINGS

These and other features and advantages of the present invention willbecome better understood with regard to the following description,appended claims, and accompanying drawings wherein:

FIG. 1A is a graph showing electrophoretic velocity vs. applied fieldfor octadecyltrichlorosilane (OTS) coated silica spheres of diameter2a=5.08 μm in the isotropic phase (65° C.) and in the nematic phase (25°C.) of E7 liquid crystal for two polarities of the elastic dipole inaccordance with the concepts of the present invention;

FIG. 1B is a graph showing electrophoretic velocity vs. applied fieldfor negatively charged OTS-coated borosilicate spheres of diameter2a=9.6 μm and positively charged didecyldimethylammonium chloride(DDMAC)-coated borosilicate spheres with a diameter of 2a=17.3 μm in thenematic phase in accordance with the concepts of the present invention;

FIG. 1C is a graph showing electrophoretic velocity vs. applied fieldfor neutral gold spheres in the nematic phase in accordance with theconcepts of the present invention;

FIG. 1D is a schematic representation of an experiment with E parallelto the overall director {circumflex over (n)}₀ (represented with solidlines) whereby the hyperbolic hedgehog is either on the left-hand sideof the sphere (upper particle, p_(x)>0) or the right (bottom particle,p_(x)<0) in accordance with the concepts of the present invention;

FIG. 2A is a graph showing the velocity vs. frequency of sphericalOTS-coated silica particles of diameter 2a=5.08 μm for triangular andsquare pulse AC fields in E7 liquid crystal (solid symbols correspond top_(x)<0 and open symbols correspond to p_(x)>0) in accordance with theconcepts of the present invention;

FIG. 2B is a schematic representation of a plurality of polarizingmicroscope images that show the movement of the OTS-coated silicaparticles of diameter 2a=5.08 μm (circles represent the spheres andcrosses represent the location of the hyperbolic hedgehog) at thefollowing times: 0 s, 16.8 s; 33.6 s; and 67.2 s after the triangular ACfield of 45 mV/μm, 100 Hz is applied in accordance with the concepts ofthe present invention;

FIG. 2C is a graph showing the velocity vs. frequency of sphericalDDMAC-treated borosilicate particles (diameter of 2a=17.3 μm) in a field10 mV/μm moving in E7 (squares), moving in a mixture with 18.7 wt % ofE7 with MLC7026-000 (circles) and OTS-treated silica particles (adiameter of 2a=5.08 μm) in a field 20 mV/μm moving in E7 (triangles),(solid curves represent the fit with Eq. 3) in accordance with theconcepts of the present invention;

FIG. 2D is a schematic representation of a plurality of polarizingmicroscope images that show the movement of the DDMAC-treated particleshaving a diameter of 2a=17.3 μm (circles represent the spheres andcrosses represent the location of the hyperbolic hedgehog) in a mixturewith 13.5 wt % of E7 with MLC7026-000 at the following times: 0 s, 11.9s, 23.8 s and 35.7 s, after a 30 mV/μm, 1 Hz electric field is appliedin accordance with the concepts of the present invention;

FIG. 3 is a schematic representation of a polarizing microscope imagethat shows electrophoretic motion of DDMAC-coated borosilicate sphereshaving a diameter of 2a=9.6 μm moving in the nematic liquid crystalMLC7026-000 in the plane perpendicular to the electric field, along the“race-track” trajectories set by a non-uniform director {circumflex over(n)}₀, with the schematic inset image showing enlarged images of twoparticles moving in opposite directions (circles represent the spheresand crosses represent the location of the hyperbolic hedgehog) inaccordance with the concepts of the present invention;

FIG. 4 is a graph showing electrophoretic velocity ν vs. E² (sinusoidalfield of 25 Hz, p_(x)>0) for OTS-coated silica spheres with a diameterof 2a=5.08 μM (circles); OTS-coated borosilicate spheres having adiameter of 2a=9.6 μm (squares); and gold spheres having a diameter of2a=10 μm (triangles), moving in E7 in accordance with the concepts ofthe present invention; and

FIG. 5 is a schematic representation of spherical particles with normalanchoring at the surface that are embedded in a uniformly-alignednematic LC with quadrupolar symmetry (A) and dipolar symmetry (B) withan electric field applied along the director {circumflex over (n)}₀, anddipolar symmetry (C) with an electric field applied perpendicular to thedirector {circumflex over (n)}₀, (director distortions shown by dottedlines), whereby the Saturn ring configuration shown in (A) preserves thefore-aft symmetry, resulting in zero electrophoretic mobility, while thehyperbolic hedgehog configuration shown in (B) breaks the fore-aftsymmetry, and (C) breaks the left-right symmetry and are responsible forthe nonlinear electrophoresis in accordance with the concepts of thepresent invention.

DETAILED DESCRIPTION

Before presenting the particular features of the present invention, thefollowing discussion presents an overview of the physical propertiesassociated with general electrophoretic motion to assist the reader inunderstanding the invention presented below.

With regard to electrophoretic motion, the electric charge of a particledispersed in a fluid is screened by a diffuse cloud of mobilecounterions, which have a charge that is opposite in sign to that of theparticles. When an electric field is applied, the counterions and theparticle move in opposite directions to one another. For a smallparticle, the effects of inertia can be neglected, and itselectrophoretic velocity v is determined by the electrostatic “pulling”force that is proportional to the applied electric field E and by theviscous drag force imparted by the surrounding fluid. The balanceresults in the linear velocity-field dependence given by Smoluchowski'sformula:

v=μE,  (1)

where μ=∈_(m)ζ/η is the electrophoretic mobility of the particle, and isa material constant that is independent of E, and is proportional to theso-called “zeta-potential” ζ, which characterizes the charge of theparticle and its spatial distribution. The electrophoretic particlemobility μ is also defined by the dielectric permittivity of the medium∈_(m), while being inversely proportional to the viscosity η of themedium. According to Eq. (1), an AC electric field with a zero timeaverage produces no net electrophoretic propulsion, as a change inelectric field polarity changes the sign of v such that theperiod-averaged displacement of the particle is zero. This is whynumerous applications of electrophoresis, including electrophoretictranslocation and separation of macromolecules, such as DNA andproteins; controlled assembly of colloidal particles, microfluidics; andelectrophoretic displays, also known as e-ink displays, rely on a DCelectric field to drive the particles. However, the use of DC fieldslead to undesirable electrochemical reactions, and as such, there is astrong interest in the electrophoretic mechanisms where the relationshipbetween v and E is non-linear. Most studies consider isotropic fluids tobe an electrophoretic medium where nonlinear behavior can be observedeither for high voltages, or for particles with special properties, suchas a patterned surface. In one aspect, the non-linear correction iscubic in the applied field such that the velocity of the particle isν=μE+μ₃E³, where μ₃ is a non-linear mobility found in nematic liquidcrystals, the details of which will be discussed below.

With regard to the present invention, electrophoretic motion iscontemplated utilizing an orientationally-ordered fluid, such as anematic liquid crystal (LC) in which the dependence v(E) has a componentquadratic in E. The dependence ν˜E² allows one to move particles even bya symmetric, sinusoidal AC field with a zero time average and modestamplitude, as the field polarity does not influence ν. The contemplatednon-linear electrophoretic motion is caused by asymmetric distortions ofthe liquid crystal orientation around the particle. The electrophoreticvelocities linear and quadratic in E have generally differentdirections, which brings about a high degree of freedom in moving theparticle in space. We demonstrate electrophoretic motion parallel(anti-parallel) to E, perpendicular to E, as well as motion alongcurvilinear tracks set by spatially varying orientation of the liquidcrystal.

To demonstrate the electrophoretic motion discussed above, aroom-temperature nematic E7 (EM Industries) liquid crystal sample thatmelts into an isotropic phase at t_(NI)=58° C. was used. The samplesrepresent a liquid crystal layer of thickness h=(50-80) μm disposedbetween two glass plates. The plates are coated with polyimide PI2555(Microsystems), which is mechanically buffed to align the liquid crystalalong the x-axis in the plane of cell. Molecular orientation in theliquid crystal is described by the director {circumflex over (n)}; sincethe medium is non-polar, {circumflex over (n)}=−{circumflex over (n)}.Unidirectional buffing implies that far away from the colloid,{circumflex over (n)}=(1,0,0)=const. In addition, the field E=(E,0,0) isparallel to the x-axis, to avoid torque on the director away from theparticles. The dielectric anisotropy of the E7 liquid crystalΔ∈=∈_(∥)−∈_(⊥)=13.8 is positive (∈_(∥) and ∈_(⊥) are the dielectricpermittivities for E parallel and perpendicular to {circumflex over(n)}, respectively), so that E aligns {circumflex over (n)} parallel toitself. Two aluminum strips separated by a gap L=5-12 mm served as theelectrodes.

Dielectric spheres of diameter 2a=(5-50)μM made of silica (BangsLaboratories), borosilicate and soda lime glass (Duke Scientific), aswell as gold spheres with a diameter of 2a=(5.5-9)μm (Alfa Aesar) werealso added to the E7 liquid crystal in small quantities (<1 wt %) toavoid aggregation. All particles produce perpendicular orientation of{circumflex over (n)} at their surfaces, with gold particles producingthis alignment once their surface has been etched with an acid. Finally,the dielectric spheres were functionalized with surfactants,octadecyltrichlorosilane CH₃(CH₂)₁₇SiCl₃ (OTS, Sigma-Aldrich), andN,N-didecyl-N-methyl-(3-trimethoxysilylpropyl)ammonium chloride[(CH₃O)₃Si (CH₂)₃N (CH₃)(CH₂)₁₈(CH₃)₂]Cl (DDMAC, Sigma-Aldrich).

Uniform DC Field

Since liquid crystals always contain some amount of ionic impurities(10¹⁵/cm³), the voltage profile across the cell is time-dependent. Toscreen the field, the ions move and build electric double layers nearthe electrodes, within a characteristic time τ_(e)=λ_(D)L/2D≈(1-5) μm,where λ_(D)=(0.1-1)μm is the Debye screening length and D=10⁻¹⁰-10⁻¹¹m²/s is the diffusion coefficient. The measurement v is performed withina few minutes after voltage has been applied, in the regime ofstationary motion, whereupon the voltage polarity is reversed and v ismeasured again. The aforementioned process was repeated three times.

Isotropic phase: In the isotropic phase, the dielectric spheres show alinear electrophoresis, with ν/E=μ=const, as shown in FIG. 1A. TheOTS-coated spheres move in the direction opposite to the electric field,with a mobility μ_(OTS) ^(l)=0.6±0.05 μm²/mV·s (for diameter of 2a=5.08μM which implies a negative zeta potential. The DDMAC-coated particlesmove in the same direction as E, with μ_(DDMAC) ^(l)=0.3±0.08 μm²/mV·s(for 2a=9.6 μm). The gold spheres do not move, μ_(Au) ^(I)=0.

Nematic phase: With a perpendicular {circumflex over (n)}, each particlegenerates a radial director configuration in the surrounding liquidcrystal, a so-called radial hedgehog of a topological charge 1. Theuniform overall alignment of the cell implies that the total topologicalcharge of the system is zero. The conservation law of topologicalcharges requires that the +1 charge of the sphere is compensated by a −1charge of additional director distortions. These are known to be ofeither one of two types: a point defect, the so-called hyperbolichedgehog, as shown in FIG. 1D, or an equatorial disclination ring. Thehedgehog configuration leads to the nonlinear electrophoresis, while thesecond configuration of quadrupolar symmetry displays only a regularlinear effect.

The pair comprised of a sphere and an accompanying hedgehog forms anelastic dipole p=(p_(x),0,0), as shown in FIG. 1D. This dipole iselastically repelled from the bounding plates of the cell, so that thedispersed particles levitate in the bulk of the cell, thus resistingsedimentation, which hinders electrophoresis in isotropic fluids. Ifthere is no field, the levitating spheres experience Brownian motionwith zero net displacement.

Once the field is applied, the particles dispersed in the liquid crystalare set into motion, with a pronounced nonlinear dependence v(E), asshown in FIGS. 1A-C. In the case of charged dielectric spheres, vchanges its sign not only when the field polarity is reversed but alsowhen |E| increases, as shown in FIGS. 1A-B. For metallic spheres, v doesnot change its direction when the polarity of E is reversed, such thatthe gold sphere always leads the way ahead of the hedgehog, as shown inFIG. 1C. The experimental data in FIG. 1 is well described, if inaddition to the linear term in Eq. (1), one uses also a term quadraticin E:

ν=μE+βE ².  (2)

For neutral gold particles μ_(Au)=0, the dependence ν(E) is parabolicwith the tip at the center of coordinates and β_(Au) ^(N)=2×10⁻³μm³/mV²·s. For dielectric particles, the linear term does not vanish(the parabolae's tips are shifted in FIGS. 1A,B) and fitting yieldsμ_(OTS) ^(N)=−0.01 μm²/mV·s, β_(OTS) ^(N)=0.53×10⁻³ μm³/mV²·s forparticle diameter of 2a=5.08 μm; μ_(OTS) ^(N)=−0.03 μm²/mV²·s,β_(OTS)=2.55×10⁻³ μm³/mV²·s for particle diameter of 2a=9.6μm, andμ_(DDMAC) ^(N)=0.07 μm²/mV²·s, β_(OTS) ^(N)=5.5×10⁻³ μm³/mV²·s forparticle diameter of 2a=17.3 μm. The μ^(N) coefficients are somewhatlower than their counterparts μ^(I) in the isotropic phase, which isexpected, as the viscosity of the E7 liquid crystal decreasesexponentially with temperature, by about a factor of 10 between 65° C.and 25° C. It should be appreciated that the β coefficients that arezero in the isotropic phase become non-zero in the nematic liquidcrystal phase.

Uniform AC Field

The quadratic term in Eq. (2) above indicates that the electrophoresisin the liquid crystal can be driven by an AC electric field, even ifthis field is symmetric and has a zero time average, as observed in FIG.2. The linear term in Eq. (2) averages to zero. FIG. 2A shows thefrequency dependencies of ν for dielectric spheres in the E7 liquidcrystal driven by AC fields of different profiles and amplitudes. The ACfield drives the particles along {circumflex over (n)}₀, parallel to p,with the sphere leading the way, as shown in FIGS. 2A-B.

Equation (2) is written for the case when the vectors v, p, and E areall parallel (or anti-parallel) to the x-axis, as shown in FIG. 1D.Generally, the dependence v(E) for nonlinear electrophoresis in a liquidcrystal involves tensorial coefficients that depend on p, so that thecomponents of the velocity ν_(i) and the field E_(j) (i, j=x, y, z) arerelated as

ν_(i)=μ_(ij) E _(j)+β_(ijk) E _(j) E _(k).  (3)

The tensor character of v(E) is manifested by the fact that β (but notμ) changes sign with p, so that the velocity components originating inthe linear (μ_(xx)E_(x)) and quadratic (β_(xxx)E_(x) ²) parts of Eq. (3)can be not only parallel, but also anti-parallel to each other, as shownin FIGS. 1A-B and FIGS. 2A-B and 2D. Another illustration comes from theexperiments with the liquid crystal MLC7026-000 (Merck) in whichΔ∈=∈_(∥)−∈_(⊥)=3.7. The negative Δ∈ allowed the application of the fieldE=(0,0,E_(z)) perpendicularly to p=(p_(x), p_(y),0) without distortingthe liquid crystal far away from the particles. By buffing the polyimidealigning layer in a circular fashion, a cell was prepared in which{circumflex over (n)}₀(x, y) was non-uniform, and which formed arace-track configuration, as shown in FIG. 3. The electric field wasapplied by using transparent indium tin oxide electrodes that confinedthe cell from the top and the bottom. The spheres moved in the (x, y)plane of the cell, perpendicular to E, following the curvilineartrajectories of the racetrack, either counterclockwise or clockwise,depending on the polarity of p, as shown in FIG. 3. The typical velocityis 1 μm/s in the field E=100 mV/μm of frequency 1 Hz. This experimentdemonstrates that the liquid crystal-based electrophoresis offers a highdegree of flexibility in the control of particle motion. The latter canbe further expanded if one uses both DC and AC electric fields, sincethe AC field contributes only to the second term in Eq. 3.

In a separate experiment, the role of dielectric anisotropy Δ∈ wasverified. In the E7 liquid crystal, Δ∈=13.8 is large, causing adielectric torque ∝Δ∈ on the director near the spheres. To minimize thistorque, E7 liquid crystal was mixed with MLC7026-000, whereupon Δ∈reduces to 1.25 at the concentration 18.7 wt % of E7 and practicallyvanishes to Δ∈=0.03 at 13.45 wt % (measured for 25° C. and 1 kHz). Theelectrophoresis in these two mixtures was similar to the case of E7liquid crystal, as shown in FIG. 2C, demonstrating that the dielectricreorientation of the director is not the prevailing driving mechanism.

The nonlinear character of AC propulsion is especially clear when ν isplotted vs. E² (for a fixed frequency of a sinusoidal profile), as shownin FIG. 4. Fitting of the data with Eq. (2) yields the values of thesame order of magnitude as in the DC case: β_(Si) ^(N)=1.5×10⁻³/m³/mV²·sfor silica particles of diameter 2a=5.08 μm, β_(BSi) ^(N)=3.4×10⁻³μm³/mV²·s for borosilicate particles of diameter 2a=9.6 μm, and β_(Au)^(N)=3.8×10⁻³ μm³/mV²·s for gold particles of diameter 2a=10 μm.

The AC and nonlinear DC electrophoretic effects are observed forspherical particles when the director distortions around them are of adipolar type, as shown in FIG. 1D. If the director distortions preservethe fore-aft symmetry, as is the case of the equatorial defect ring,shown in (A) of FIG. 5, these effects vanish. To produce the defectstructure with an equatorial ring, the recipe of Gu and Abbott wasfollowed, namely, by using shallow cells with the separation between theplates that is close to the diameter of particles. The AC field causedback-and-forth linear electrophoresis of the particles, but no netpropulsion.

DISCUSSION

Thus, it should be appreciated that the mechanisms of the AC andnon-linear DC electrophoresis in a liquid crystal medium are rooted inthe type of LC director distortions, which violate the fore-aft (orleft-right) symmetry of the liquid crystal. In contrast, forelectrokinetic phenomena in isotropic fluids, the broken symmetry of theparticles themselves can lead to a non-linear DC and AC electrophoresiswith ν∝E². Induced-charge electrophoresis (ICEP) was experimentallydemonstrated for anisotropic quartz particles and for Janus sphericalparticles (comprised of two semispheres with different properties) in anisotropic fluid. However, the critical difference in liquid crystalelectrophoresis is that particle motion is caused by the broken symmetryof the medium, rather than by the particle itself, as shown in FIG. 5.That is, the difference in operation of the present invention is that inan isotropic fluid, the particle must be asymmetric for AC effect totake place. In the present invention, the use of an anisotropic medium,such as a liquid crystal, allows a particle, such as an ideal sphere tobe propelled, as the needed symmetry breaking is provided by the carrieranisotropic medium itself.

Consider an uncharged metallic (gold) particle in a liquid crystal. Oncethe electric field is switched on, the mobile ions of the liquid crystalstart to move in two opposite directions. These ions cannot penetratethe surface of a particle, and thus accumulate at its opposite sides.The field-induced ionic clouds attract the “image charges” from withinthe conducting sphere, thus producing a field-induced Debye screeninglayer. The field-induced non-uniform zeta potential can be estimated asζ_(ind)˜aE. In the steady state, the tangential component of E drivesthe mobile ions, and thus the fluid, from the poles of the spheretowards its equator. The directionality of this ICEP flow does notchange with the field reversal, as shown in FIG. 5. If the particle andthe surrounding medium have mirror-image symmetry, the slip velocityproduces no electrophoretic propulsion, as illustrated in FIG. 5A for asphere with an equatorial defect ring. The hedgehog configuration, shownin FIG. 5B-C, breaks this symmetry, such that the flows on the oppositesides of the sphere are not symmetric and give rise to anelectrophoretic velocity ˜ν˜(aE)E˜aE², where one power of E sets up theinduced charge clouds and the second power drives the resultant flow.

This qualitative picture suggests that in Eq. (2), β=δ∈_(m)a/η formetallic particles, where δ is the dimensionless factor characterizingthe medium asymmetry; δ=0 in FIG. 5A and δ≠0 in FIGS. 5B-C. If theparticle is dielectric, the characteristic length should be λ_(D)instead of a, so that β=δ∈_(d)λ_(D)/η, where ∈_(d) is the permittivityof the particle. For gold spheres with a diameter of 2a=10 μm,estimating, η=0.1 Pa·s, and average permittivity ∈_(m)=10∈₀ for E7, andassuming δ=1, one finds β=5×10⁻³ μm³/mV²·s close to β_(Au) ^(N)=4×10⁻³μm³/mV²·s measured in the AC field. For dielectric spheres, one findsβ=0.5×10⁻³ μm³/mV²·s using ∈_(d)=5.8∈₀, λ_(D)=1 μm and δ=1 of the sameorder as the experimental values.

For metallic particles, the director field asymmetry is the onlymechanism of electrophoretic propulsion ν=βE², since the permanent zetapotential ζ is zero, as shown in FIG. 1C. For dielectric spheres, theelectrophoretic velocity is determined by both the linear and quadraticterms ν=μE+βE², as shown in FIGS. 1A-B. In the AC field, the linear termaverages out and ν∝E² for both the metallic and dielectric particles, asshown in FIG. 4.

The following discussion presents how ν depends on the frequency of thesinusoidal AC field, as shown in FIG. 2, following the considerationsfor isotropic fluids. The ICEP velocity is controlled by two timescales; 1) a characteristic charging time τ_(c)=λ_(D)a/D (for aconductive sphere) and τ_(e)=∈_(m)λ_(D) ²/∈_(d)D (for a dielectricsphere); and 2) the characteristic electrode charging timeτ_(e)=λ_(D)L/2D. In the order of magnitude, τ_(c)=10⁻² s and τ_(e)=10²s,with λ_(D)=1 μm and D=10⁻¹¹ m²/s.

For moderate fields and thin counterionic clouds, λ_(D)<<a<<L, the bulkAC field is controlled by τ_(e):

${{E_{o}(t)} = {\frac{V_{o}}{L}{\cos ( {\omega \; t} )}{{Re}\lbrack {\frac{\; \omega \; \tau_{e}}{1 + {\; \omega \; \tau_{e}}}^{{- }\; \omega \; t}} \rbrack}}},$

and the time dependent polarization of the sphere is proportional to

${{Re}( \frac{^{{\omega}\; t}}{1 + {\omega\tau}_{c}} )}.$

Combining these two results, one obtains the frequency dependence of theelectrophoretic velocity:

$\begin{matrix}{{v(\omega)} = {v_{o}{\frac{\omega^{2}\tau_{e}^{2}}{( {1 + {\omega^{2}\tau_{c}^{2}}} )( {1 + {\omega^{2}\tau_{e}^{2}}} )}.}}} & (4)\end{matrix}$

Equation (4) describes the experimental dependencies ν(f), where f=ω/2π,as shown in FIG. 2C. The velocity increases as ω² when w is low, but atthe high ω, ν decreases since the ions cannot follow the rapidlychanging field. All three experimental dependencies in FIG. 2C werefitted with practically the same values of parameters in Eq. 4, namelyτ_(c) in the range (0.005-0.015) s, τ_(e) in the range (43-53) s andL=10 mm.

SUMMARY

Thus, electrophoretic motion, including highly-nonlinear motion, ofparticles in an orientationally-ordered nematic liquid crystal has beendemonstrated. The particles can be driven either by a DC or AC electricfield, regardless of whether their zeta potential is finite (dielectricspheres) or zero (metallic spheres). The liquid crystal electrophoresisis much richer than its isotropic counterparts, as it adds new degreesof freedom in particle manipulation. In an isotropic fluid, theelectrophoretic particle must be electrically charged (have a nonzeropermanent zeta potential) or be asymmetric in shape or in surfaceproperties. Neither of these requirements needs to be satisfied in theliquid crystal case, as the particle can be of any shape, including ahighly-symmetric sphere, and can be of any charge state. Thus, theliquid crystal as an electrophoretic medium provides additional degreesof freedom in the electrically-controlled manipulation of particles. Thecomponents of velocity that originate in the linear and quadratic termsof Eqs. (2) and (3) need not be parallel to each other, and theparticles can be moved in any direction in 3-D space. The describedphenomenon of liquid crystal electrophoresis offers new perspectives forpractical applications where a highly-flexible, precise, and simplecontrol of particle (or cargo) placement, delivery, mixing, or sortingis needed. Examples include microfluidic devices, electrophoreticdisplays and their hybrids with the conventional liquid crystal displays(LCDs) that have been already explored for the case of linearelectrophoresis. The practical potential of liquid crystal-basedelectrophoresis is further expanded by the fact that the trajectoriesand velocities of particles can be controlled not only by the frequencydependent linear and quadratic mobilities in Eqs. (2) and (3), but alsoby the spatially-varying director field {circumflex over (n)}₀ (r) thatcan be used as a curvilinear “rail” to transport the particles, as shownin FIG. 3.

In addition, while the described mechanism of liquid crystal basedelectrophoresis is based on the use of thermotropic liquid crystals asthe carrier medium, other liquid crystal types may also be used, such assolvent-based lyotropic liquid crystals, and the like. Lyotropic liquidcrystals, such as chromonics, are compatible with materials ofbiological origin, thus allowing nonlinear electrophoresis to be used toseparate molecules of biological origin, such as DNA, proteins, andparticles such as vesicles, bacteria, and viruses, which is highlydesirable. Biological molecules can also be transported in thermotropicliquid crystals, either in an encapsulated form, or in their nativeform, especially if they are hydrophobic.

The use of liquid crystals as an electrophoretic medium, as contemplatedby the present invention, thus provides the unique ability to transferparticles in any type of AC or DC electric field and to do so forparticles of arbitrary shape (including spheres), and arbitrary electriccharge (even when this charge is zero). It has been demonstrated thatelectrically-neutral gold particles of vanishingly small charge stillmove in an AC field, as it is the ionic nature of the nematic fluid thatsupplies the charges. Furthermore, a high electric field is not requiredto cause the nonlinearity, as it is rooted in the symmetry of themedium; therefore, the nematic LC carrier can be used in electrophoretictransport of very soft objects or particles in a low electric field. Theeffect is of importance for fundamental understanding of electrokineticphenomena in complex fluids and provides an approach to colloidalassembly, separation, microfluidic and micromotor applications, as well.

Therefore, one advantage of the present invention is that due to thequadratic or non-linear dependence ν˜E², electrophoretic motion in aliquid crystal as contemplated by the present invention occurs naturallyunder an action of an alternating current (AC) electric field, with azero time average, which is in contrast to the linear electrophoresisthat is observed in regular isotropic fluids, where the highly desirableAC field transport is impossible because of the linear dependence ν˜E,whereupon the AC field shifts the particles back and forth with no netpropulsion. Another advantage of the present invention is that theelectrophoretic effect in the liquid crystal allows one to transportboth charged and neutral particles, even when the particles themselvesare perfectly symmetric (spherical), thus enabling new approaches incolloidal assembly, separation, microfluidic and micromotorapplications. Yet another advantage of the AC-driven electrophoresis ofthe present invention is that it can be used in separation of proteins,DNA molecules, viruses and microbes, vesicles and various otherbiological entities. Another advantage of the present invention is thatone can create steady particle flows/motions, while avoiding both“memory effects” associated with irreversible absorption of ions andchemical reactions caused by DC field near the electrodes, which isimportant in prolonging the operating life of electrophoretic e-ink ande-paper displays. Still another advantage of the present invention isthat 3-D control of particle trajectories (through the control of linearand quadratic components of velocity) is enabled, allowing for sortingand display applications, as it can allow one to move the particles ofdifferent types in different directions.

Although the present invention has been described in considerable detailwith reference to certain embodiments, other embodiments are possible.Therefore, the spirit and scope of the appended claims should not belimited to the description of the embodiments contained herein.

1. A method of electrophoretic movement of a particle comprising:providing a nematic liquid crystal between a pair of plates, saidnematic liquid crystal having a director field in which the orientationof said director field proximate to said plates has a first alignmentorientation; disposing a particle having a surface in said nematicliquid crystal, such that the orientation of said director fieldproximate to said surface has a second alignment orientation differentfrom said first alignment orientation to thereby form distortions insaid director field around at least part of said surface of saidparticle; subjecting said liquid crystal to an electric field, such thatsaid distortions around said particle cause the velocity of mobile ionsin said liquid crystal moving between said plates to be asymmetric; andmoving said particle through said nematic liquid crystal.
 2. The methodof claim 1, wherein said plates are at least partially transparent. 3.The method of claim 1, wherein said first alignment orientation of saiddirector field is substantially parallel to said plates.
 4. The methodof claim 1, wherein said nematic liquid crystal is a thermotropic liquidcrystal.
 5. The method of claim 1, wherein said nematic liquid crystalis a lyotropic liquid crystal.
 6. The method of claim 1, wherein saidnematic liquid crystal has a positive, negative or zero dielectricanisotropy.
 7. The method of claim 1, wherein said particle comprises anOTS-coated sphere.
 8. The method of claim 1, wherein said particlecomprises a DDMAC-coated sphere.
 9. The method of claim 1, wherein saidparticle is a metallic sphere.
 10. The method of claim 1, wherein saidparticle is a dielectric sphere.
 11. The method of claim 1, wherein saidparticle is asymmetric in shape.
 12. The method of claim 1, wherein saidparticle is conductive.
 13. The method of claim 1, wherein said particleis formed of polymeric material.
 14. The method of claim 1, wherein saidparticle is a carbon nanotube.
 15. The method of claim 1, wherein saidparticle is a biologic particle selected from the group consisting ofproteins, DNA molecules, viruses, microbes, and vesicles.
 16. The methodof claim 1, wherein said electric field is a DC electric field.
 17. Themethod of claim 1, wherein said electric field is an AC electric field.18. The method of claim 1, wherein said second alignment orientation ofsaid director field is substantially perpendicular with respect to saidsurface of said particle.
 19. The method of claim 1, wherein said secondorientation of said director field is tilted at an angle with respect tosaid surface of said particle.
 20. A method of electrophoretic movementof a particle comprising: providing a pair of plates; disposing apolyimide layer on one of said plates; buffing said polyimide layer in apredetermined direction; disposing a nematic liquid crystal between saidplates, said nematic liquid crystal having a director field, such thatthe orientation of said director field proximate to said polyimide layerhas a first alignment orientation that is in substantial alignment withsaid predetermined direction; disposing a particle having a surface insaid nematic liquid crystal, such that the orientation of said directorfield proximate to said surface has a second alignment orientation thatis different from said first alignment orientation to thereby formdistortions in said director field around at least part of said surfaceof said particle; subjecting said liquid crystal to an electric field,such that said distortions around said particle cause the velocity ofmobile ions in said liquid crystal moving between said plates to beasymmetric; and moving said particle through said nematic liquid crystalalong said predetermined direction.
 21. The method of claim 20, whereinsaid plates are at least partially transparent.
 22. The method of claim20, wherein said first alignment orientation of said director field issubstantially parallel to said plates.
 23. The method of claim 20,wherein said nematic liquid crystal is a thermotropic liquid crystal.24. The method of claim 20, wherein said nematic liquid crystal is alyotropic liquid crystal.
 25. The method of claim 20, wherein saidnematic liquid crystal has a positive, negative or zero dielectricanisotropy.
 26. The method of claim 20, wherein said particle comprisesan OTS-coated sphere.
 27. The method of claim 20, wherein said particlecomprises a DDMAC-coated sphere.
 28. The method of claim 20, whereinsaid particle is a metallic sphere.
 29. The method of claim 20, whereinsaid particle is a dielectric sphere.
 30. The method of claim 20,wherein said particle is asymmetric in shape.
 31. The method of claim20, wherein said particle is conductive.
 32. The method of claim 20,wherein said particle is formed of polymeric material.
 33. The method ofclaim 20, wherein said particle is a carbon nanotube.
 34. The method ofclaim 20, wherein said particle is a biologic particle selected from thegroup consisting of proteins, DNA molecules, viruses, microbes, andvesicles.
 35. The method of claim 20, wherein said electric field is aDC electric field.
 36. The method of claim 20, wherein said electricfield is an AC electric field.
 37. The method of claim 20, wherein saidsecond alignment orientation of said director field is substantiallyperpendicular with respect to said surface of said particle.
 38. Themethod of claim 20, wherein said second orientation of said directorfield is tilted at an angle with respect to said surface of saidparticle.
 39. A liquid crystal cell comprising: a pair of plates adaptedto be coupled to a power source; a liquid crystal material disposedbetween said plates, said liquid crystal material having a directorfield in which the orientation of said director field proximate to saidplates has a first alignment orientation; and a particle disposed withinsaid liquid crystal material, said particle having a surface andconfigured such that orientation of said director field proximate tosaid surface has a second alignment orientation different from saidfirst alignment orientation to thereby form distortions in said directorfield around at least part of said surface of said particle; whereinwhen power is applied to said plates by said power source, saiddistortions cause the velocity of mobile ions in said liquid crystalmaterial moving between said plates is asymmetric, causing said particleto move.
 40. The method of claim 39, wherein said plates are at leastpartially transparent.
 41. The method of claim 39, wherein said firstalignment orientation of said director field is substantially parallelto said plates.
 42. The method of claim 39, wherein said nematic liquidcrystal is a thermotropic liquid crystal.
 43. The method of claim 39,wherein said nematic liquid crystal is a lyotropic liquid crystal. 44.The method of claim 39, wherein said nematic liquid crystal has apositive, negative or zero dielectric anisotropy.
 45. The method ofclaim 39, wherein said particle comprises an OTS-coated sphere.
 46. Themethod of claim 39, wherein said particle comprises a DDMAC-coatedsphere.
 47. The method of claim 39, wherein said particle is a metallicsphere.
 48. The method of claim 39, wherein said particle is adielectric sphere.
 49. The method of claim 39, wherein said particle isasymmetric in shape.
 50. The method of claim 39, wherein said particleis conductive.
 51. The method of claim 39, wherein said particle isformed of polymeric material.
 52. The method of claim 39, wherein saidparticle is a carbon nanotube.
 53. The method of claim 39, wherein saidparticle is a biologic particle selected from the group consisting ofproteins, DNA molecules, viruses, microbes, and vesicles.
 54. The methodof claim 39, wherein said electric field is a DC electric field.
 55. Themethod of claim 39, wherein said electric field is an AC electric field.56. The method of claim 39, wherein said second alignment orientation ofsaid director field is substantially perpendicular with respect to saidsurface of said particle.
 57. The method of claim 39, wherein saidsecond orientation of said director field is tilted at an angle withrespect to said surface of said particle.